1. Field of the Invention
The present invention relates to laser annealing apparatus and method for performing an annealing process on an object to be processed by irradiating the object with a linear beam.
2. Description of the Related Art
In a process for manufacturing thin film transistors used for liquid crystal semiconductor devices, a laser annealing apparatus is used in order to modify a substrate (a semiconductor substrate or a glass substrate) by irradiating the substrate with a laser beam. The modification of the substrate includes crystallization of an amorphous semiconductor thin film such as an amorphous silicon film and activation of impurities added to a semiconductor film.
FIG. 5 is a schematic view of a conventional laser annealing apparatus 30. In the laser annealing apparatus 30, a laser beam 32 emitted from a laser 31 is shaped into a beam with a linear cross section through a beam shaping optical system 33. This linear beam scans a substrate 34, which is an object to be irradiated, in a short-axis direction of the beam (an X direction in the figure); thus, a desired region over the substrate 34 is irradiated with the laser beam (see, for example, Patent Document 1: Japanese Published Patent Application No. 2006-287183). Accordingly, the substrate 34 is modified.
In such a laser annealing apparatus 30, in general, the beam position is fixed and the substrate 34 is mounted on a substrate stage. Then, the beam scan is performed while moving the substrate in the short-axis direction of the linear beam. Here, the stability in directivity and the stability in positioning of the laser beam 32 are not zero and its emission position or emission angle is deviated from that of a reference beam. In FIG. 5, an angle-deviated beam 35 and a position-deviated beam 36 are schematically illustrated. If such a position deviation or angle deviation occurs in the short-axis direction of the linear beam, the beam position on the surface to be irradiated moves back and forth in terms of time in the short-axis direction (X direction) of the beam hereinafter this phenomenon is called “drift”). As a result of the drift occurring in the short-axis direction of the beam, the irradiated region on the substrate 34 includes a region where the irradiation time with the laser beam 32 is relatively long and a region where it is relatively short.
Accordingly, irradiation unevenness that can be observed even with human eyes appears along the short-axis direction of the beam (the substrate moving direction) as shown in FIG. 6. Since this unevenness is observed over a region of about several millimeters, it is considered that the beam is deviated in angle or position for about several seconds. If thin film transistors are manufactured using a substrate including such unevenness, transistor characteristics vary because crystal quality is different in accordance with the unevenness.
In response to such problems, Patent Document 2 (Japanese Published Patent Application No. 2000-42777) has disclosed a technique for minimizing the drift.
FIG. 7 illustrates a structure of a drift correction device 41 of a laser process apparatus 40 disclosed in Patent Document 2. In FIG. 7, the drift correction device 41 includes a laser 42 including a pair of reflection mirrors 43 and 44 which forms an optical resonator; a beam shaping unit 51 for changing a laser beam emitted from the laser 42 into a linear beam with a narrow cross section; a beam center position detection device (a line sensor 45 and a profile position calculation device 46) for detecting a center position in a width direction of the beam after passing through the beam shaping unit 51; and an orientation control mechanism (a drift amount calculation device 47, a mirror angle calculation device 48, a rear mirror rotation drive device 49, and a front mirror rotation drive device 50) which, in the case where the detected center position P is deviated by the beam drift, changes the orientation θf and/or θr of at least one reflection mirror of the optical resonator so as to eliminate the deviation ΔP. Further, in FIG. 7, the laser process apparatus 40 includes a beam homogenizer 52 for shaping and condensing a laser beam into a linear beam with a narrow cross section on a surface of a substrate 53.
With the use of the drift correction device 41 having such a structure, the deviation ΔP is calculated based on the beam center position detected by the line sensor 45. The orientation θf and/or θr of at least one reflection mirror of the optical resonator is changed so as to eliminate this deviation ΔP. Accordingly, the drift of the laser beam on the surface of the substrate 53 is suppressed.